When an electron beam in a regular cathode ray tube device is scanned in horizontal direction, the distance from the deflection center to a central portion of the effective picture area differs from the distance to a short side portion of the effective picture area. Because of this difference, the scanning distances of the electron beam for the same deflection angle are different. Usually, an S-correction is performed to correct the resulting image distortion. In an S-correction, the slanted portions of the saw-tooth-shaped deflection current waveform are formed into an S-form.
The amount of the S-orrection is set to a value where the east-west pincushion distortion at the short side of the effective picture area, that is, at the left and right edges of the effective picture area, becomes zero. In recent years, however, ever larger cathode ray tube devices and ever flatter cathode ray tube device screens have brought about an even larger difference between the distance from the electron beam deflection center to a central portion of the effective picture area and the distance from the electron beam deflection center to a short side portion of the effective picture area. Thus, as is illustrated with vertical lines in FIG. 6(a), inner pincushion distortion 1a and 1b occurs in the right and left intermediate portions located between the short sides Y.sub.a of the effective picture area A and the y-axis, which is parallel to the short sides Y.sub.a and passes through the center of the effective picture area A. As illustrated with horizontal lines in FIG. 6(b), inner pincushion distortion 1e and 1f occurs in upper and lower intermediate portions located between the long sides X.sub.a of the effective picture area A and the x-axis, which is parallel to the long sides X.sub.a and passes through the center of the effective picture area A. Particularly when using the cathode ray tube device for applications such as CAD, this causes straight to lines bend into curved lines, and circles to bend into ellipses, which may be very irritating and obstruct work efficiency.
In conventional cathode ray tube devices such as disclosed, for example in Publication of Unexamined Japanese Patent Appilication No. Hei 5-83585, the deflection circuit comprises an additional circuit such as a modulation transforming circuit. This circuit overlaps a horizontal signal with a parabola wave signal that is synchronized with a vertical signal, so that the distortion amount .delta.' of the vertical inner pincushion distortion 1a and 1b is decreased.
However, in such conventional cathode ray tube devices, which have a deflection circuit comprising an additional circuit such as a modulation transforming circuit, the necessary deflection power is about 10% higher than in cathode ray tube devices without such additional circuits. Moreover, the cost of such a device will increase by the cost of the additional circuit.
The present invention has been developed to overcome the problems of the prior art. It is a purpose of the present invention to provide a cathode ray tube device in which the amount of inner pincushion distortion in the effective picture area is decreased without an increase of the deflection power and device costs.